She told the men, dressed in dark suits and carrying leather portfolios, that they would have to wait until the doors opened at 11. That was when they flipped out their badges: They were federal agents investigating reports of "anti-American activity" at the tiny art gallery.

To FBI special agent Terrence Donahue and Steven Smith of the Secret Service, it was a routine mission to check out one of the more than 435,000 tips they have received since Sept. 11.

To Ms. Huanca, whose gallery was opening "Secret Wars," an exhibit on US covert operations and government secrets, it was something else. "What's anti-American about freedom of speech?" the docent blurted out.

The incident, which ended after an hour of questioning, represents more than just a disturbing day for one museum staffer. Across the US, growing numbers of Americans are facing similar interrogations - apparently, they say, because they have criticized the government, President Bush, or the war on terrorism.

Not everyone is bothered by the inquiries. Indeed, by responding to a torrent of tips federal agents are doing exactly what many Americans want them to do.

But as the nation mounts a zealous campaign against domestic terror, some observers say federal agencies are walking a delicate line between checking out leads and trampling on free speech.

"If the FBI is investigating art exhibits at museums, then the line has been crossed," says First Amendment scholar David Cole at Georgetown University in Washington. "The FBI should investigate any credible leads where federal criminal activity may be undertaken. But it should avoid investigating any political conduct."

The rise in doorstep inquiries reflects, in part, a new law-enforcement reality. Suddenly, it may seem hard to know who might be the next to steer a plane into a building. It also reflects raw math. There are simply many more tips to check.

"Remarks made toward the president in an antagonistic way are checked out by the Secret Service. That's always been the case," says Jill Spillman, an FBI agent detailed to the Justice Department. "The FBI checks out [possible] domestic terrorism." She says the people visited are under no obligation to answer questions and are not necessarily viewed as suspects.

But Attorney General John Ashcroft's post-September policy is that each tip be looked into. While not every tip leads to a face-to-face visit, surprise encounters with federal agents are leaving some Americans feeling their privacy has been violated - and that their speech has made them targets of official scrutiny.

For example, A.J. Brown, a student at Durham Technical Community College in North Carolina, faced 40 minutes of grilling by two Secret Service agents and a Raleigh police officer in her doorway (she wouldn't let them come in, and they had no search warrant). By her account, they said they were investigating a tip that she had "un-American material" in her apartment. From the doorway, they took particular note of a poster of George W. Bush holding a noose. It read: "We hang on your every word," referring to his unflinching support of the death penalty as governor of Texas.

Then there's San Franciscan Barry Reingold, who was awakened from his afternoon nap by a buzzing intercom on Oct. 23. He called down to the street to find out who it was. "The FBI," was the response. He buzzed the two men up, but decided to meet them in the hall. "I was a little bit shaken up," says Mr. Reingold. "I mean, why would the FBI be interested in me, a 60-year-old retired phone company worker?"

When they asked if he worked out at a certain gym, he realized the reason behind the visit. The gym is where he lifts weights - and expounds on his political views.

Since Sept. 11, the sessions have been heated. Once, he recalls, "discussion turned to [Osama] bin Laden and what a horrible murderer he was. I said, 'Yeah, he's horrible and did a horrible thing, but Bush has nothing to be proud of. He is a servant of the big oil companies, and his only interest in the Middle East is oil.' "

Some fellow weightlifters called Reingold a disloyal American. One, apparently, called the government.

So it was that two agents were standing in his hall. "They said, 'You know you are entitled to freedom of speech.' And I said, 'Thank you. That ends our conversation.' " When Reingold closed his door, he heard one of the agents say: "But we still need to do a report."

As the overheard comment suggests, the FBI and Secret Service view many of these checkups as a routine, almost innocuous, part of their job.

Still, the task has taken on fresh relevance after the terrorist attacks.

"Just because we [talked] to ABC Flight School doesn't mean they did something wrong," says Robert Doguim of the Houston FBI. "But how irresponsible of us would it be if we didn't talk to someone?"

He says the Art Car Museum and its exhibit (which had been planned months before Sept. 11) were deemed "not dangerous" after the agents' visit.

But to Huanca, the face-off seemed unnecessary and intimidating. She says the G-men puzzled over each art installation, sneering and saying things like, "What's that supposed to mean?"

Drawing conclusions from cases like this is tricky, since the reality could involve more, or less, than either side tells the media. But Barry Steinhardt, associate director of the American Civil Liberties Union in Washington, finds the anecdotal evidence deeply troubling.

"All of this speaks to the new McCarthyism, where political dissent is being equaled to treason," Mr. Steinhardt says. "It's a very frightening trend: that people are doing nothing more than expressing the very freedoms that we are fighting to preserve - and find themselves with the FBI at their door."